The present invention relates to centrifugal pumps in general, and more particularly to improvements in multistage centrifugal pumps of the type wherein the housing has an inlet extending axially and an outlet extending radially of the pump shaft and the stages include a full-admission standard-impeller-type first stage and at least one partial-admission self-priming regenerative stage downstream of the first stage.
Pumps of the above outlined character are often utilized when the conditions for admission of a liquid medium into the inlet of the pump housing are far from optimal, e.g., when the liquid medium must be sucked from a source which is located at a level well below that of the inlet of the pump housing or when the inlet is located at a level below but close to the level of the source of supply of the liquid medium. The first stage enhances the so-called suction behavior of the pump, namely the NPSH (net positive suction head) value, but without considering the efficiency and the pump head. The regenerative stage renders it possible to increase the pressure of the conveyed fluid medium at a loW RPM and with a small number of stages.
German Offenlegungsschrift No. 15 28 826 discloses a centrifugal pump of the above outlined character wherein the diameter of the radial impeller of the first stage is smaller than the diameter of the star wheel of the regenerative stage. This enhances the overall design of the pump but the pump cannot be used for the conveying of liquefied gases because the self-priming operation is ensured only in the presence of an auxiliary liquid.
In many countries, the laws, rules and regulations pertaining to safety in connection with the storage of liquefied gases are so stringent that the source of supply (such as a tank or another vessel) of liquefied gaseous medium must be (or preferably is) confined well below the ground level. In view of such location of the source, the pump which is used to draw the liquefied gaseous medium must be installed close to the source (and hence at a level well below the ground), i.e., such pump must constitute an elongated submersible motor pump which is not only complex and expensive due to the need for reliable encapsulation of the pump and the motor therefor but is also unaccessible or hard to reach for inspection, repair or replacement. Alternatively, the pump is a suction pump which is mounted at a level above or on the ground. Such pump must be a self-priming pump which operates without cavitation. The heretofore known pumps fail to meet these requirements because excessive quantities of liquid are evacuated therefrom in response to each interruption of liquid flow through the housing. The upper level of the liquid is then located at the lowermost point in the inlet of the pump housing so that the liquid which remains in the housing does not suffice to ensure reliable evacuation of gases from the suction pipe and adequate priming when the motor is started again.
At least in many instances, a pump which is used to convey a liquefied gas is further required to convey a medium which is devoid of gas bubbles or other forms of inclusions of gaseous media. On the other hand, and if the pump is called upon to convey a liquefied gas, it is practically impossible to prevent at least partial conversion of liquefied gas into the gaseous phase, especially if the inlet of the pump housing must draw liquefied gas under less than optimum circumstances. Evaporation of some liquefied gas entails an increase of pressure which, in turn, entails liquefaction of some of the gaseous phase in the region of the inlet and in the suction pipe which delivers liquefied gas from a source that is confined in the ground. Nevertheless, it is necessary to provide such pumps with gas separators which are supposed to ensure that the housing receives a stream which is devoid of any bubbles.
European Pat. No. 45 483 discloses a centrifugal pump wherein the housing contains a rather large liquid reservoir which is installed between the first stage and the regenerative stage. The liquid which is confined in the reservoir is intended to ensure reliable self-priming in response to renewed starting of the pump. However, it has been found that the provision of a reservoir between the first stage and the regenerative stage does not guarantee reliable and predictable functioning of the pump, especially as regards the operation following renewed starting after a relatively long or even after a short interval of idleness. It happens frequently that the entire supply of liquefied gas in the reservoir is reconverted into a gas when the pump is idle for an extended interval of time or when the confined medium is not maintained at an optimum temperature. For example, if the pump is installed above the ground level and is heated by sunshine to a temperature well above that of the liquefied gas in the underground vessel, the entire supply of liquefied gas in the reservoir which is provided in the interior of the pump housing is likely to be reconverted into a gas so that the regenerative stage is incapable of ensuring a predictable self-priming action. This necessitates the utilization of complex and expensive electronic controls which monitor the aggregate state in the reservoir and ensure the admission of liquefied gas when the supply of liquefied gas in the reservoir is depleted below the minimum acceptable value. The electronic controls are reliable only as long as they receive electrical energy but they fail completely in the event of a blackout or another failure of the source of electrical energy and/or of the means for connecting the controls with such source.